Threshold values occur a lot in science but these days few of my students know what a threshold is. Here it is in the photos: a doorstep. It is the entrance - in science it is the value at which something starts - just as if you were going into the house. An example would be the threshold frequency for the photoelectric effect. Above a certain frequency, any light shone on a metal will release electrons from the surface which can allow a current to flow across a specially designed gap in the circuit. Below that frequency, no electrons are released and nothing happens. So that frequency is the threshold frequency. Go beyond it and the action starts.
